Play in windowed mode with 2 different accounts. How to two box or better referred to as Dual Client. You need to go into Final Fantasy XI: Online configurations select the Screen Size tab. From there select a resolution with triple digit by triple digit resolution. Example;
640 x 480
720 x 480
800 x 600
once you have selected from any of these resolutions; make sure you have start in window mode checked.
Open Final Fantasy XI shortcut twice then log into both accounts.
Note you don't have to have smaller resolution you could have bigger. But it helps to control both accounts better in smaller resolution.